The processes used at the present time for production of large series of pieces of composite materials schematically employ two methods for preparation of the base product and two methods for conversion of the base product into finished pieces.
The preparation of the base product may be carried out:
1. by calendering--the product obtained is in the form of a preimpregnated sheet or SMC (sheet molding compound);
2. by mixing--the product obtained is in the form of a viscous mass of heterogeneous appearance; the "sauerkraut" or BMC (bulk molding compound).
The conversion of the base product into the finished piece is effected:
1. by compression in vertical presses. The material is first measured and then placed into an open heated mold (case of SMC and BMC);
2. by injection in horizontal injection presses. The BMC material is forced under pressure into a closed and heated mold (conventional injection) or injected under reduced pressure into a heated and semiclosed mold (injection/compression).
The mechanical properties of the molded product may vary depending on the mode of preparation of the material and the conversion method chosen for the molding within a broad range from 1000 kg/cm.sup.2 of bending strength for an injected BMC to 1800 kg/cm.sup.2 for the same BMC converted by compression and 2100 kg/cm.sup.2 for a compression molded SMC using the same material formulation in each case.
It is advantageous to improve the method of the BMC injection to utilize the industrial advantage associated with "injection" equipment. BMC injection is the easiest and most economical process to apply industrially but yields pieces with the poorest mechanical properties.